Illustrated herein are embodiments relating to a method and apparatus for reducing intercolor bleed to improve print quality. They find particular application in addressing intercolor bleed problems and will be described with particular reference thereto. However, it is to be appreciated that these embodiments are also amenable to other like applications.
An ink jet printer prints an image by printing a plurality of pixels on a recording medium such as paper or a transparency. The pixels are printed by ejecting drops from the ink jet printheads forming spots also referred to as pixels P on the recording medium as shown in FIG. 1. Adjacent pixels are printed to overlap each other to ensure complete coverage of the printed image region R. The pixels are often printed having a spot size diameter D of approximately the square root of two times the pixel spacing S to ensure overlap of diagonally adjacent printed pixels. At a boundary B between pixels of two colors, P1 and P2, this overlap of different colored printed pixels mixes inks of different colors resulting in the phenomenon known as intercolor bleed.
For color ink jet printers which print on plain paper, one of the challenges is to achieve a proper balance in ink properties which allows penetration into the paper at a sufficiently rapid rate so that pools of different colored inks in adjacent areas do not appreciably intermix.
Intercolor bleed is most noticeable for images which contain sharply defined borders between two patches of ink of different colors. Such patterns frequently occur in business graphics, for example. When intercolor bleed occurs, instead of the desired sharply defined border, the border appears ragged and fuzzy.
For color ink jet printers which print on plain paper, intercolor bleed has been reduced by striving to achieve a proper balance in ink properties which allows penetration into the paper at a sufficiently rapid rate to reduce the pools of different colored inks in adjacent areas thus reducing intermixing. However, the penetration of ink should not occur so rapidly so as to allow edge sharpness to be dominated by a random pattern of paper fibers. Often, the cyan, magenta, and yellow inks are formulated using penetrants or surfactants as constituents to enable penetration into the paper within less than a second, i.e., so that ink at the surface is no longer substantially mobile. For sharp black text and high optical density in black printing, however, it is preferable to formulate the black ink so that it less rapidly penetrates (e.g., penetrates in seconds to tens of seconds). This is called medium dry black ink. Intercolor bleed can be particularly problematic at the boundary between black pixels and pixels of other colors.
Other techniques for reducing intercolor bleed include techniques for altering the image by deleting pixels or printed drops at the borders between colors. This gives the two adjacent patches a relief zone so that wet pools are less likely to come into contact and intermix.
In this regard, several patents teach various pixel modification algorithms to minimize intercolor bleed. For example, U.S. Pat. No. 6,361,144 to Torpey et al. relates to a method for processing color image data to reduce intercolor bleeding in an image printed on a recording medium. U.S. Pat. No. 6,290,330 to Torpey et al. relates to a method of processing color image data for printing in an ink jet printer to maintain edge quality in an image recorded on a recording medium. U.S. Pat. No. 6,183,062 to Curtis et al. provides a method for processing color image data to maintain edge quality in an image recorded on a recording medium. In addition, U.S. Pat. No. 6,343,847 to Torpey et al. relates to a method for processing color image data to determine if a target pixel is within a boundary region near a border.
However, printing algorithms which perform pixel or drop deletion may produce undesirable printing artifacts on certain types of images, such as pictorial images. It has been demonstrated that pixel management algorithms work significantly better for spot sizes corresponding to printing resolutions of 400 spi and above.
In addition, printing using printheads for printing different sized spots is known. For example, U.S. Pat. No. 5,745,131, entitled “Gray Scale Ink Jet Printer” by G. Kneezel, W. Burger, S. Harrington, D. Ims, and J. Stephany, describes a pattern of laying down dots for gray scale in which a first array of ejectors deposits ink spots of a first size on a first grid pattern, and a second array of ejectors deposits ink spots of a second size on a second grid pattern which is offset from the first grid pattern. The two arrays are also fired in time such that placement of the different sized spots is also offset in the scan direction. Other embodiments of this type of printhead are described in U.S. Pat. No. 6,402,280, entitled “Printhead with Close-Packed configuration of Alternating Sized Drop Ejectors” by G. Kneezel, D. Mantell, J. O'Neill, T. Tellier and S. Harrington and U.S. Pat. No. 6,375,294, entitled “Gray Scale Fluid Ejection System With Offset Grid Patterns of Different Size Spots” by G. Kneezel.
For some ink jet printers, printhead operating temperatures can affect the size of the ejected ink drops and thus the spot size diameter D. During operation, the printhead typically heats up, increasing the size of the ink drops ejected from printhead nozzles thereby producing larger spots on the recording medium. U.S. Pat. No. 6,422,677, entitled “Thermal Ink Jet Printhead extended droplet volume control” by N. Deshpande, et al. teaches pre-pulsing the nozzle heaters to keep spot size constant as printhead temperature varies.
However, these techniques of printing different drop sizes (and others), do not address the problem of intercolor bleed in a region of a printed image having a border between two colors.